Conventional biaxially oriented polypropylene (BOPP) films are commonly laminated to monolayer breathable blown films of various types of polyethylenes, ethylene copolymers or ionomers. BOPP films are known to provide excellent transparency, thermal resistance, excellent gloss, and high tensile strength. The conventional BOPP film typically has not been modified and as such has high levels of crystallinity that reduce the transfer of gases such as oxygen and carbon dioxide through the film. Typically, BOPP film is supplied in a thickness between 0.4 mil to 0.7 mil. This film typically provides a barrier of 260 cc/100 in2/day (0.4 mil) to 149 cc/100 in2/day (0.7 mil). Obviously, it is necessary to reduce the thickness of the BOPP film in order to provide adequate oxygen transmission. This reduction in thickness can be undesirable, as the film generally becomes more susceptible to edge tearing as the film is decreased in gauge. This edge tearing can be counterproductive for the converter. In addition, the BOPP would preferably be treated on one surface to accept printing inks readily. This BOPP film is reverse printed on the treated surface and subsequently adhesively laminated to the monolayer blown film. Again, as film thickness is decreased, the film is generally more difficult to print by the converter in terms of maintaining print registration due to the thin film's lower tensile properties and a propensity to stretch more easily under tension. Typically, an antifog coating is applied to the monolayer film after lamination. This laminated film is then formed into a package to enclose product such as fruit, vegetables, or fresh cut salad that require breathability.
There are a variety of applications, such as food packaging, that require packaging film that have excellent tensile, optical properties and gas transmission properties. However, there are several problems with current packaging films.
U.S. Pat. No. 6,232,402 (DeMeuse '402) discloses biaxially oriented polypropylene film comprising a mixture of 75-92% by weight isotactic polypropylene, 5-15% by weight low density polyethylene, and 3-10% by weight of an olefin heteropolymer containing polypropylene and at least one other 2-4 carbon alpha olefin. The invention cited examples claiming oxygen transmission values in the range of between 206.0-285.6 cc/100 in2/day. These oxygen transmission values are quite low as compared to the present invention. In addition, the presence of low density polyethylene and olefin heteropolymer in the formulation, decreases the thermal properties of the BOPP film that are necessary during sealing.
U.S. Pat. No. 6,410,136 (DeMeuse '136) discloses a biaxially oriented film comprised of a mixture of 10%-60% by weight of isotactic polypropylene, 10%-25% by weight of low density polyethylene and about 15%-80% by weight of an olefin heteropolymer containing polypropylene and at least one other alpha olefin of 2 to 4 carbon atoms. The invention cited examples claiming oxygen transmission values in the range of between 323.3-589.6 cc/100 in2/day. These oxygen transmission values are in the range of the present invention, however the high levels of heteropolymer and low density polyethylene decreases the thermal properties of the film that are necessary during sealing.
U.S. Pat. No. 6,395,071 (Niijima) discloses a breathing film which contains a block copolymer composition comprising crystalline polypropylene and propylene alpha olefin random copolymer having 45-85% by weight of propylene unit-containing on the weight of copolymer. The film thickness is between 10 and 100 microns and having an anitbacterial agent coated thereon or included therein. The invention cited examples claiming oxygen transmission values in the range of between 781.3-800.8 cc/100 in2/day. These oxygen transmission values are very good, however the tensile modulus values are in the range of from 122,000-126,000 psi, which is very low for the application.
U.S. Pat. No. 6,485,817 (Demeuse '817) discloses a transparent composite oriented film comprising a first layer and at least a second layer, said first layer consisting essentially of an ethylene/propylene copolymer including less than 10% by weight ethylene, said at least second skin layer being a high tensile modulus polypropylene. The invention examples indicate oxygen transmission values in the range of 215.2-363.7 cc/100 in2/day. These oxygen transmission values are lower than the present invention.
U.S. Pat. No. 6,348,271 (Nakata) discloses a multilayer film having a thickness of 10-100 um, wherein the film comprises at least one layer obtained by processing and drawing a block copolymer composition in at least monoaxial direction, and the block copolymer composition comprises 93-30% by weight of a propylene polymer component and 7-70% by weight of a ethylene/propylene copolymer. This polymer is essentially a block copolymer where the ethylene/propylene copolymer content is tailored to balance the film transparency, tensile properties and oxygen transmission of the film. The example cited by this patent indicates oxygen transmission values in the range of 244-762 cc/100 in2/day. However, as the oxygen transmission increases, the tensile properties are also reduced dramatically.
These prior art structures use either ethylene/propylene copolymers, polypropylene block copolymers including ethylene/propylene rubber, or three component blends with low density polyethylene. These methods of increasing oxygen transmission are successful; however, each type sacrifices some desired film attribute in order to improve the oxygen transmission.
U.S. patent application Ser. No. 10/738,984 (Moreau), which is incorporated herein by reference, discloses a high oxygen transmission film composition using a blend of propylene homopolymer, propylene—containing impact copolymer, and an alpha-olefin propylene—containing thermoplastic olefin. Skin layers are added to this primary layer to provide stiffness and gloss, and in the examples cited, the skin layers are isotactic propylene homopolymers.
In light of the deficiencies of the prior art, this invention provides several advantages, among others, not only to increase the oxygen transmission of the film, but also to maximize gloss, tensile strength, and thermal resistance while minimizing haze. This BOPP film with balanced properties performance satisfies the total film requirements where the prior art structures do not fully satisfy all of these criteria.